The immune system is remarkable in its ability to mount responses against a wide array of antigens and to respond with enhanced vigor to antigens encountered in the past. This exaggerated recall immune response or immune memory is a central concept in immunology and it forms the basis of vaccination against infectious diseases. Immune memory is mediated by memory lymphocytes that persist in the host long after resolution of the infection or antigenic insult. To study immune memory, we have developed a novel and powerful transgenic mouse model system. In this system, both effector and memory pools of T lymphocytes are indelibly tagged with the marker, human placental alkaline phosphatase (PLAP) by an irreversible genetic recombination event. Our long-range goal is to understand how immune memory is generated, regulated and maintained. The objective of this application, which is the first step in pursuit of this goal, is to further validate the transgenic mouse model system and to ask fundamental questions about how CD8 T cell memory is generated. The central hypothesis of this application is that antigen-specific effector and memory CD8 T cells can be unambiguously identified. We will use this model system to probe lymphocytic choriomeningitis (LCMV) virus-induced immune responses and the differentiation pathway of immune memory generation. The objective of this application will be accomplished by pursuing five specific aims: (1) to compare expression of PLAP and LCMV peptide/MHC class I tetramer-binding in CD8 T cells during the acute and memory phases of the LCMV immune response, (2) to determine whether PLAP+ effector CD8 T cells are the sole precursors of memory CD8+ T cells, (3) to compare proliferative and survival differences between PLAP+ and PLAP+ CD8+ effector T cells at the peak of acute anti-viral response (4) to determine when the decision to become CD8 memory T cells is made and (5) to determine the mechanism behind PLAP marking of a subset of effector CD8 T cells in the primary immune response. It is our expectation that the resultant approaches will identify key events involved in the generation of CD8 T cell memory. The research will be of significance because what is learned from these studies will contribute to broader understanding of how vaccines that provide long-term protective immunity can be rationally designed.